<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <div id="root"></div>
    <script src="../../js/react.js" charset="utf-8"></script>
    <script src="../../js/react-dom.js" charset="utf-8"></script>
    <script src="../../js/browser.min.js" charset="utf-8"></script>
    <script type="text/babel">

    class Clock extends React.Component {
        constructor(props) {
            super(props)
            this.state = {date: new Date()}
        }

        componentDidMount() {
            this.timerID = setInterval(
                () => this.tick(),
                1000
            )
        }

        componentWillUnmount() {
            clearInterval(this.timerID)
        }

        tick() {
            this.setState({
                date: new Date()
            })
        }

        render() {
            return (
                <div>
                    <h1>Hello, world!</h1>
                    <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
                </div>
            )

        }
    }

    ReactDOM.render(
        <Clock />,
        document.getElementById('root')
    );
    </script>
</body>
</html>
